Effect of Te doping on structural and superconducting properties of FeSe superconductor

In the present work, we have studied the effect of doping Te at the Se-site on the superconducting properties, such as transition temperature (T-c), upper critical field (H-c2) and irreversibility field (H-irr) of FeSe. The polycrystalline samples have been prepared via solid state reaction route with nominal compositions FeSe1-xTex (x = 0.0, 0.25, 0.50 and 0.75). The X-ray diffraction results show the tetragonal alpha-FeSe phase with the p4/nmm space group symmetry in all the samples. The values of H-c2(0) estimated from Werthamer-Helfand-Hohenberg (WHH) and Ginzburg-Landau (GL) theories are found to follow the same trends and maximum H-c2(0) is found for the composition x = 0.50. The irreversibility field, H-irr and activation energy, U-0 were also calculated to see the vortex motion behavior in the samples.